Coalition spokesman Col Thomas Collins said a large cargo truck in the US convoy had suffered a mechanical failure, hitting the cars at a busy junction.

"This was a tragic accident and we deeply regret any deaths or injuries resulting from this incident," he said, adding that a full investigation was under way.

Hundreds of Afghans gathered after the accident, chanting "Death to America" and "Death to Karzai".

They pelted the US military vehicles with stones before scattering when the shooting began.

Hundreds of protesters took to the streets

Some eyewitnesses say the US troops shot at protesters, while others say it was the Afghan police, who had come to the aid of the under-siege convoy. Some say it was both.

The US military said there were "indications" that at least one of the vehicles in the convoy "fired warning shots over the crowd".

At the height of the violence, 21 European Union diplomats and staff had to be rescued from their compound by British Royal Marines working for the Nato-led International Security Assistance Force (Isaf).